Temperature has significantly negative effect on structural performance, this situation is sure to be even worse for support structure of offshore wind turbine (OWT) in harsh ocean environment. In this study, the daily temperature effects on a support structure of jacket-type OWT are investigated using numerical simulation. The basic theory and method of thermal analysis for structure are briefly introduced. The finite element (FE) models for thermal analysis of the support structure of an OWT are constructed. Then, the time-dependent thermal boundary conditions are determined using the meteorological parameters of a typical sunny day. Accordingly, the thermal boundary conditions are applied on the FE model. Subsequently, the transient heat-transfer analysis is performed for structural temperature calculation. At last, the time-dependent structural temperature variation and distribution of the support structure are discussed. The results show that the temperature effects are considerable and have obvious daily-cycle property. The solar radiation and air temperature are the major influential factors on the structural temperature behavior of the steel tube tower. The temperature effects should be seriously considered in the design and maintenance of the support structure of OWT.